FIG. 1 shows a conventional Gilbert type active mixer.
Equation 1 expresses an operation of the active mixer.
                                          I            out                                I                          i              ⁢                                                          ⁢              n                                      =                              1            π                    ⁢                      {                                                            cos                  ⁡                                      (                                                                  ω                                                  i                          ⁢                                                                                                          ⁢                          n                                                                    +                                              ω                        LO                                                              )                                                  ⁢                t                            +                                                cos                  ⁡                                      (                                                                  ω                                                  i                          ⁢                                                                                                          ⁢                          n                                                                    -                                              ω                        LO                                                              )                                                  ⁢                t                                      }                                              [                  Equation          ⁢                                          ⁢          1                ]            
An output signal lout is converted to have a required frequency for ωin+ωLO or ωin−ωLO by switching operations of transistors Q+ and Q−.
The mixing operations are not influenced by a bias voltage Vbias. The bias voltage Vbias is used to determine an operating point to allow the transistors Q+ and Q− to operate properly.
FIG. 2 shows an embodiment of a conventional current steering variable gain amplifier.
An input signal lin is steered by an output through a transistor Qp or a supply through a transistor Qn and controlled by a voltage Vc.
A current gain is determined by Equation 2. in below
                                                        I              out                                      I                              i                ⁢                                                                  ⁢                n                                              =                      1                          1              +                              exp                ⁡                                  (                                      -                                                                  V                        c                                                                    V                        T                                                                              )                                                                    ,                              V            T                    =                      kT            q                                              [                  Equation          ⁢                                          ⁢          2                ]            
In general, in order to guarantee a dB linearity of the voltage Vc, dependence on process, temperature, and pre-distortion has to be removed. Circuits for removing the dependence on process, temperature, and pre-distortion have been widely proposed (see: “A highly-Integrated SiGe BiCMOS WCDMA Transmitter IC”, by Abdellatif Bellaouar, Michel Frechette, Ahmed R. Fridi, Sherif H. K. Embabi, International Solid-State Circuits and Systems, 2002, 14.5).